This invention relates to a weighing apparatus and particularly to a weighing apparatus for weighing loads in a hazardous environment. In particular, the present invention relates to weighing apparatus which uses load cells to provide a weight indication.
A load cell is a transducer which provides an electrical signal having a magnitude related to the force applied to the cell. Load cells are used in apparatus for weighing a variety of loads including large transport vehicles such as railroad cars and trucks. A typical load cell weighing apparatus has a weighing platform which is supported at a number of spaced points by load cells. The load cells are connected to a remote instrument which powers the load cells and provides a weight indication in response to the electrical signal provided by the load cells.
Weighing apparatus of the above type is sometimes used in environments wherein explosions may result from electrical discharges. Usually the weighing instrument, itself, is situated within a safe area, and the weighing platform and the load cells are located in the unsafe environment. The voltage and current levels present in the load cells are normally low enough that there is no danger of electrical discharge. If the load cell were to fail, however, the voltages and/or currents in the load cells could reach dangerous levels. To limit the voltages and/or currents in the load cells to safe levels, circuits known as "intrinsic safety barriers" are interposed in each electrical line between the load cell and the instrument.
Intrinsic safety barriers normally permit the free flow of electrical signals between the safe and unsafe areas. The barriers block excessive voltages and currents, however. The intrinsic safety barriers thus keep the electrical energy in the circuits in unsafe areas so low that ignition of combustion mixtures due to an electrical discharge cannot occur even under adverse or abnormal conditions.
When intrinsic safety barriers are provided, a signal voltage drop occurs across each barrier. In many types of apparatus this voltage drop is of no consequence. It can be important, however, in apparatus wherein signals affected by the voltage drop are already at low levels. Weighing apparatus falls into this category.
Specifically, the use of intrinsic safety barriers constrains the number of load cells which may be used in the weighing apparatus. The current drawn by the load cells is related to the number of load cells employed. As the number of load cells is increased, current drawn through the intrinsic safety barriers increases. Because of this, no more than, for example, four load cells can be used in a weighing apparatus without exceeding the current limitation imposed by the intrinsic safety barrier. Unfortunately four load cells is too few to accurately weigh a large distributed weight, as is required in some applications.
The change in output voltage accompanying a minimum graduation weight change also diminishes as the number of load cells increases. If only one load cell is used, for example, its output voltage may change by 18 microvolts (uv) for a 20 lb change in applied weight. If ten load cells are used, the same weight change will produce only a 1.8 uv change in output signal. The instrument whose task it is to measure this voltage may, however, be unable to accurately measure low level changes of this nature. Thus, the number of load cells which can be used is also limited by the sensitivity of the instrument to which they are connected.